The present invention is related to an apparatus for the thermal treatment of substrates, especially semiconductor substrates, in a chamber in which the substrates are placed upon support elements.
An apparatus of this type for the treatment of substrates is known, for example, from DE-A-198 21 007 of the same applicant. With this apparatus, the support elements for carrying semiconductor wafers are rigidly connected to a rotary plate that is rotated for the homogenization of the thermal treatment of a semiconductor wafer.
An alternative carrier construction for the apparatus of the aforementioned type is illustrated, for example, in FIG. 1. Here a wafer 1 is placed upon conically shaped support studs 2 which are also called pins. The pins, along with their holding means 4, are rigidly secured to a carrier frame 3. The pins have conically shaped support tips in order to keep the contact surfaces between the support elements and the wafer small, and to thus minimize a transfer of heat from the wafer to the support element and to minimize a thereby resulting cooling off of the wafer in the region of the contact surface. Unfortunately, these support tips have the drawback that they break off easily and thereby result in undesired particles. Furthermore, the support tips leave behind mechanical impressions in the wafer that rests upon them, since at high temperatures the wafer material is relatively soft. Especially with wafers having relatively large diameters, such as, for example, 300 mm wafers, which have a weight of 130 g instead of 50 g for wafers having a 200 mm diameter, the pressure forces between support tips and wafer increase and thus the problem of mechanical impressions increases.
These mechanical impressions furthermore increase during the thermal treatment of the wafer due to the thermal expansion or distortion of the wafer and a thereby resulting relative movement between wafer and support pin. During the heating up of a wafer from room temperature to 1000° C., the thermally caused increase in diameter is, for example, in the region of 1 mm. Consequently, the tips of the support pins scrape over the surface of the wafer and at those locations leave behind long scratches, as illustrated by way of example in FIGS. 2a and 2b. FIGS. 2a and 2b show defect locations on the back side of a semiconductor wafer, which were produced by the known support pins during the thermal treatment of the wafer. The mechanical impressions and the scratch widths can be examined, for example, by cleavage. With this method, the wafer is broken up into predefined pieces, whereby a line of fracture or a fault line extends through a location that is to be examined. By means of an SIRD process (scanned infrared depolarization), one obtains an indication about the magnitude of the existing stresses at the contact locations. With this method, a double break produced by elastic deformation is measured that results with many transparent and isotropic materials due to deformation.
The mechanical impressions and scratches, as well as inhomogeneity of the wafer temperature during the thermal treatment, lead to dislocation errors, so-called slip lines, in the crystalline structure of the semiconductor wafer. Although these occur on the underside, i.e. the support side of the wafer, during the thermal treatment of the wafer they can, if the thermal stress is sufficiently great, propagate to the upper surface and damage or adversely affect the structures that are disposed on the upper surface. Such slip lines are visible, for example, from structural etching.
For a good thermal treatment of the semiconductor wafer, a homogeneous temperature distribution over the wafer is necessary. As already mentioned, due to the contact with the support pins, however, there results a localized cooling off of the wafer in the region of the contact, which leads to inhomogeneity of the temperature distribution upon the wafer. This problem was solved in the past in that the contact surface between support pin and wafer was kept small, which, however, worsened the aforementioned problem of scratching. In practice, the support pins were therefore positioned in an edge region of the wafer with a spacing of approximately 1 to 10 mm from the edge of the wafer. This was intended to ensure that the slip lines caused by the support forces did not damage the electronic components or structures disposed upon the surface of the wafer. However, this edge support results in the problem that during the thermal treatment the wafer sags, which again enhances the formation of dislocation errors or slip lines.
U.S. Pat. No. 5,817,156 furthermore discloses a substrate treatment apparatus that is provided with holding pins that are movable perpendicular to the plane of the substrate in order to position various regions at varying distances relative to a heating plate. However, with these holding pins the aforementioned problems of scratching occur that result from a relative movement between substrate and pin as a consequence of thermal expansion of the substrate.
Proceeding from the aforementioned state of the art, it is an object of the invention to provide an apparatus of the aforementioned type according to which the formation of scratches upon the surface of the wafer is reduced.